The presence of actin and myosin-like proteins in platelets has led to the attractive generalization that the interaction of these proteins provides the generating forces for the various platelet functions such as pseudopod formation, aggregation and clot retraction. In light of the presence of two forms of platelet actin and the observation that motion in many non-muscle cells is partially regulated by rapid aggregation/disaggregation of actin monomers, suggests that platelets also contain regulatory proteins similar to those described for muscle and non-muscle cells. The stimulus for pseudopod formation, acting at the plasma membrane level, induces actin polymerization which can be found extending to the tip of the pseudopod. The studies proposed include biochemical analysis of the contractile, regulatory, anchoring and auxiliary proteins in controlling the equilibrium between polymerizable and unpolymerizable platelet actins. The interaction between these proteins will be studied by a micro-technique that involves the binding of proteins to polystyrene latex particles and by the technique of actin nucleotide exchange. By the former technique, the affinity of bound proteins to other proteins present in solution can be determined kinetically. Concomitantly, the ultrastructural organization of these proteins will be studied by labeling platelets with tagged antibodies prepared against each of these proteins. The immunolocalization of these proteins will be studied in platelets both at rest and following stimulation by an aggregating agent. The purpose of these studies is to elucidate the participation of the contractile proteins in the sequence of events leading to pseudopod formation, platelet/platelet interaction and thrombus formation in damaged vessels. How contractile proteins are arranged at rest and rapidly reorganized within the cells after stimuli are transmitted from the platelet membrane is fundamental to an understanding of normal and deranged platelet function.